1. Field of the Invention
The present invention relates to the use of a polymer containing one or more calcium compounds in the polymer matrix, particularly paint sludge powders, which are admixed in Portland cement as reactive expansive additives in providing shrinkage compensating concrete. In particular, the present invention relates to rod reinforced shrinkage compensating concrete containing the expansive additive.
2. Description of Related Art
Shrinkage compensating concrete has found particular application in parking structures and other buildings, pavements, bridge deck overlays, patching of pavements (potholes), and water storage tanks to control cracking caused by shrinkage movements in concrete structures. If inexpensive expansive additives could be found, then the usage would be more widespread. The present market for expansive additives for use in concrete is several thousand tons per year. Expansive additives for use in concrete are currently sold at prices of several hundred dollars per ton.
One of the major disadvantages of Portland cement concrete is its susceptibility to tensile cracking when volume contractions associated with drying shrinkage are wholly or partially restrained as discussed in "Properties of Concrete" by A. M. Neville, Longman Scientific and Technical (1991). Shrinkage cracking is unsightly and destroys the integrity of concrete. Special costly allowances must be made in design and construction of concrete pavements and other structures in order to reduce the restraint against shrinkage movements.
Volume expansions during moist curing in ordinary Portland cements are very small. If greater expansions could be reached during hardening (e.g., through the use of expansive additives), the effects of contractions that occur upon drying could be offset. The development of expansive additives for the production of "shrinkage compensating concrete" dates back about 50 years. Commercial production began in the United States in the 1960's, and current consumption is about 500,000 tons annually (Mindess, S. and Young, J. F., "Concrete" Prentice-Hall, Inc., Englewood Cliffs, N.J., pp. 38-45 (1981)).
All the variants of present day expansive additives are based on the formation of ettringite (calcium sulfoaluminate hydrate) in considerable quantities during the first few days of curing. Mehta (Mehta, P. K., Concrete, Structure, Properties, and Materials 6, pp 206-207 (1986)) suggests that the ettringite is able to attract a large number of water molecules which cause interparticle repulsion, thus producing an overall expansion of the system. Proprietary expansive admixtures are also commercially available which confer expansive characteristics in curing concrete. Thus, expansive admixtures are assuming increasingly important roles in the production of shrinkage-compensating concrete.
In field applications of shrinkage compensating concrete, the potential expansion produced by ettringite formation is controlled by the use of ordinary steel reinforcement. Steel restrains the overall bulk expansion of the concrete, thereby converting expansion into a slight prestress within concrete. To resist expansive forces, steel is placed in tension in the concrete and concrete into compression. About 25 to 100 psi (170 to 700 kpa) of compressive stress is generated, which is generally sufficient to ensure that drying will not cause tensile cracking. The concrete either remains in compression or develops relatively small tensile stresses. The exact prestress will depend on total expansion and the amount of reinforcement. Some restraint of expansion may also occur through subgrade friction in slabs or formwork.
Prestress is developed only in the direction and the vicinity of the reinforcing steel. Thus, correct positioning of the steel is important to provide correct restraint, and misplaced reinforcement could lead to lack of adequate prestress or complications such as warping due to differential expansions. These are design considerations.
Expansive additives have been used in a wide variety of concrete structures. One of the more frequent uses in the United States has been in parking structures, to prevent water leaks that can cause damage to cars. The largest such structure is the parking building at O'Hare International Airport in Chicago, which used 120,000 yd.sup.3 (90,000 m.sup.3) of shrinkage-compensating concrete. In concrete pavements using expansive additives, the shrinkage control joints can be combined with thermal expansion control joints and other construction joints. The elimination of shrinkage control joints is an attractive advantage in the laying of pavements. The largest paving job to date using the expansive additives is probably at the Love Field Airport at Dallas-Fort Worth, where more than 150,000 yd.sup.3 (1,150,000 m.sup.3) of shrinkage-compensating concrete was used in taxiways. Use of shrinkage-compensating concrete in bridge deck overlays helps to minimize cracking and control the corrosion of steel in bridge decks. In patching of concrete pavements, where shrinkage can cause separation, shrinkage compensating concrete has found widespread application. Expansive concrete can also be used in structures where water-tightness is an important requirement, such as water storage tanks, swimming pools and ice rinks. Shrinkage-compensating concrete has also been used in tilt-up construction, where residual prestress helps elements withstand the stresses imposed during lifting, and the separation between elements is minimized.
A process for producing dry powders from overspray in painting of various products, particularly vehicles, is described in Environmental Information 13 to 17 (May 1992). This publication suggests uses for the powder in cement blocks on roofing mastics essentially as a filler. There was no suggestion that such paint powders could be used for an expansive concrete. The drying process for the powder includes a heated screw conveyor which dries the powder. Volatile organic compounds are removed from the dried paint along with water during the drying.
Numerous polymer compositions have been suggested for use in concrete as fillers. Such uses of polymers are taught by Japanese Patent No. 60033276 (Abstract; 1985), for instance where foamed styrene is used for paver stones. Numerous other patents describe polymer powders used as fillers. These polymers are essentially insert and do not cause the concrete to expand.